The present invention relates to a binder for a magnetic recording medium and also to a magnetic recording medium using the binder.
Magnetic recording mediums are widely used for sound recording tapes, video tapes and floppy disks. A typical magnetic recording medium has a magnetic layer stacked on a non-magnetic support. The magnetic layer is made of a ferromagnetic powder dispersed in a binder.
Magnetic recording mediums are required to be on high levels in various characteristics such as electromagnetic transducing characteristics, running durability and running performance. More specifically, audio tapes for recording and reproducing music are demanded to have higher original sound reproducing capability. Video tapes are demanded to be excellent in original picture reproducing capability and other electromagnetic transducing characteristics.
Thus, magnetic recording mediums are demanded to have excellent running durability, as described above, as well as excellent electromagnetic transducing characteristics. To obtain excellent running durability, an abrasive and a lubricant are generally added to the magnetic layer.
However, it is necessary in order to obtain the required running durability by an abrasive to increase the amount of abrasive added to the magnetic layer to some extent. Consequently, the content of ferromagnetic powder lowers. When an abrasive having a relatively large particle size is used in order to obtain excellent running durability, the abrasive is likely to project from the surface of the magnetic layer. Accordingly, it is likely that the improvement in the running durability by addition of an abrasive will result in deterioration of the electromagnetic transducing characteristics.
In order to improve the running durability by adding a lubricant, it is necessary to increase the amount of lubricant added. Consequently, it is likely that the binder will be plasticized and hence the durability of the magnetic layer will deteriorate.
A binder, which is one of the principal components of the magnetic layer, also performs an important roll in the improvement of the durability and electromagnetic transducing characteristics, as a matter of course. Examples of binders which have heretofore been used include vinyl chloride resins, cellulose resins, urethane resins, acrylic resins, etc. However, these conventional binders involve the problems that the resulting magnetic layer is inferior in wear resistance and that members of the magnetic tape running system are contaminated.
To solve these problems, a method wherein the hardness of the magnetic layer is increased by using a rigid binder has been adopted. However, an increase in the hardness of the magnetic layer causes the magnetic layer to become markedly brittle, which gives rise to problems such as occurrence of a dropout due to contact between the magnetic recording medium and a magnetic head, and deterioration of still characteristics.
As another prior art for solving the above-described problems, a magnetic recording medium has been disclosed in Japanese Patent Application Laid-Open (KOKAI) Nos. 62-134819 (1987) and 62-208423 (1987). The disclosed magnetic recording medium comprises a vinyl chloride copolymer having a hydroxyl group that is not directly coupled to the main chain (through an alkyl or alkyl ester group) and a polar group and contains no vinyl alcohol as a copolymer component. It is stated in the above-mentioned literature that since the vinyl alcohol, which is used as a copolymer component, is produced through a saponification process, the polymer that contains the vinyl alcohol hardly changes with time, and hence the magnetic recording medium has a minimal change with time and is superior in durability, and that since the vinyl chloride copolymer has a hydroxyl group and a polar group, the magnetic recording medium is also superior in the dispersibility of the ferromagnetic powder and provides excellent electromagnetic transducing characteristics.
With the above-described magnetic recording medium, however, sufficiently excellent durability and electromagnetic transducing characteristics cannot be obtained because the strength of the magnetic layer is not sufficiently high and the smoothness of the magnetic layer surface is not satisfactory. Accordingly, the prior art cannot be said to be a sufficiently excellent magnetic recording medium when used as an S-VHS tape or an 8-mm video tape, for example, which are required to have an extremely high smoothness and excellent electromagnetic transducing characteristics.
Further, it is disclosed in Japanese Patent Application Post-Exam Publication No. 63-55549 (1988) that a modified polyurethane resin containing a multivalent OH group and SO.sub.3 M (wherein M is an alkali metal or a quaternary ammonium ion) is employed as a magnetic coating composition in order to improve the durability of the coating film. The publication states that the use of such a binder enables improvements in the surface gloss, blocking tendency, adhesive properties, etc.
There are conventional binders comprising polyurethane as follows: A polyurethane binder employing a polyester polyol as a polyol and having --SO.sub.3 M is disclosed in Japanese Patent Application Post-Exam Publication No. 58-41565 (1983). A polyurethane binder obtained from a polycarbonate containing a hydroxyl end group and a diisocyanate is disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 59-198530 (1984). A polyurethane binder formed from a polycaprolactone containing a carboxyl group is disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 62-201918 (1987). A polyurethane binder formed from a polyether containing a carboxyl group is disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 61-190717 (1986).
These polyurethane binders exhibit excellent characteristics, which are inherent in polyurethane, but they are unsatisfactory in terms of the dispersibility of the ferromagnetic powder and the long-term shelf stability and also unsatisfactory in terms of the durability under temperature and relative humidity conditions over a wide range.
The polyol components of these polyurethane binders all have a hydrophilic segment, for example, an ester linkage, an ether linkage, a carbonate linkage, etc. The polarity of such a hydrophilic segment is not so strong as that of --SO.sub.3 M or --COOH group. Therefore, the hydrophilic segment has no function of adsorption on the surface of the ferromagnetic powder. However, in a solution the hydrophilic segment may form a weak hydrogen bond, which will reduce the spread of the polyurethane molecular chain adsorbed on the surface of the magnetic material. For this reason, the adsorption volume of the binder around the ferromagnetic powder is small, and the dispersibility and dispersion stability lower. In addition, it is considered that the hydrophilic segment in the polyol increases the dependence of the mechanical properties of the magnetic coating film on the temperature and relative humidity and hence increases the dependence of the running durability on the temperature and relative humidity and also lowers the long-term shelf stability.
It is an object of the present invention to provide a binder for a magnetic recording medium which has extremely high dispersibility and which is superior in long-term shelf stability and in durability under temperature and relative humidity conditions over a wide range, and also provide a magnetic recording medium using the above-described binder.